It is known that the speed of an object can be determined by transmitting a laser pulse at the object and measuring the Doppler frequency shift of the light backscattered from it. The frequency shift as a function of the velocity V of the target is ##EQU1## where f.sub.s is the Doppler frequency shift and .lambda. is the wavelength of the laser beam. Thus, for example, using a laser operating at 852 nm (852.times.10.sup.-7 cm) a target receding at a velocity of 100 cm/sec would shift the wavelength of the reflected fight by 2.3 MHz. Four such laser devices are discussed by Menzies in "Doppler Lidar Atmospheric Wind sensors: A Comparative Performance Evaluation for Global Measurement Applications from Earth Orbit"; Applied Optics, Vol. 25, No. 15, Aug., 1, 1986. These proposed systems utilize either a heterodyne detection system or a Fabry-Perot filter to measure the Doppler shift.
In U.S. Pat. No. 5,267,010, Kremer and Korevaar disclose a device for measuring wind speed. The device utilizes a pulsed, frequency locked laser source and two Faraday atomic line filters to measure wind speeds to an accuracy of 1 meter per second. The filter transmission spectra feature sharp peaks with sides whose position in frequency can be adjusted by design. Doppler shifts in light reflected from aerosols and particles in the wind can be determined by tuning one of the two filters to provide increasing transmission with increasing frequency and tuning the other filter to provide decreasing transmission with increasing frequency. Portions of the outgoing beam and the reflected beam are passed through each of the two filters and the transmitted power measured for each filter. The wind speed can be calculated from the measurements.